Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. sextus, a, um, num. ord. adj. [sex], the sixth, Plaut. Ps. 4, 2, 5: sextus ab urbe lapis, Ov. F. 2, 682: sextus decimus ab Hercule, Vell. 1, 6, 5: hic annus sextus, postquam ei rei operam damus, Plaut. Men. 2, 1, 9; id. Most. 4, 2, 41: sexto decimo anno, Cic. Rep. 2, 33, 57: sextus locus est, etc., id. Inv. 1, 53, 102: sextus decimus (locus), id. ib. 1, 56, 109; Tac. A. 1, 17: sexta decima legio, id. ib. 1, 37 al.: sexta decima (sc. hora), Mart. Cap. 6, § 696; for which also, in one word: post sextumdecimum annum, the sixteenth, Liv. 30, 19: abdicat die sextodecimo, id. 4, 34: sextodecimo Calendas Jan., Col. 11, 2, 94.
In gram.: sextus casus, the ablative case, Quint. 1, 4, 26.